Regional Carbonate Deposition of the Pitkin Limestone (Chesterian): Washington and Crawford Counties, Arkansas Robert E

Journal of the Arkansas Academy of Science

Volume 30 Article 35

1976 Regional Carbonate Deposition of the Pitkin Limestone (Chesterian): Washington and Crawford Counties, Arkansas Robert E. Tehan University of Arkansas, Fayetteville

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Recommended Citation Tehan, Robert E. (1976) "Regional Carbonate Deposition of the Pitkin Limestone (Chesterian): Washington and Crawford Counties, Arkansas," Journal of the Arkansas Academy of Science: Vol. 30 , Article 35. Available at: http://scholarworks.uark.edu/jaas/vol30/iss1/35

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Regional Carbonate Deposition of the PitkinLimestone (Chesterian): Washington and Crawford Counties, Arkansas ROBERT E.TEHAN Geology Department, University of Arkansas Fayetteville, Arkansas 72701 ABSTRACT The Pitkin Limestone overlies black shale of the Fayetteville Formation and is the youngest Mississippian unit in the Paleozoic succession of northwest Arkansas. Five major fades have been delineated withinthe formation by apetrographic examination of samples collected from 17 measured sections: (1) oolith facies, (2) bioclast facies, (3) nodular limestone-shale facies, (4) mudstone facies, and (5) lime mud mound facies. The distri- bution of these facies in the Pitkin Formation suggests that Fayetteville terrigenous sedimentation was succeeded by the deposition of widespread oolith shoals and skeletal blanket sand bodies across the northern Arkansas structural platform. Sparse accumulations of lime mud formed in quiet protected areas within the coalescing carbonate complex. Increasing water depth and decreasing turbulence as Pitkin sedimentation proceeded allowed the establishment of bryozoan and blue-green algal communities. The entrapment and stabilization of carbonate mud by these organisms promoted mound development and growth. Scattered oolith shoals formed adjacent to the growing mounds in more turbulent water. AAound development was terminated in shallower water by extensive oolith and by the appearance of extensive skeletal sand accumulations in more turbulent water as regression was initiated.

INTRODUCTION Lime mudstone units range from thin to thick-bedded and commonly are in the lower 10 feet of the formation. A few shale The PitkinLimestone, the youngest Mississippian (Chesterian) unit stringers are associated withthese beds. They appear dense and show exposed in northwest Arkansas, crops out along the northern conchoidal fracture on fresh surfaces. They contain very few fossils. escarpment of the Boston Mountains from Batesviile, Arkansas, to The lime mud mounds are massive and have a gnarled or lumpy Muskogee, Oklahoma. Stratigraphically the Pitkin conformably appearance. They are usually lenticular. The mound core is overlies the Fayetteville Formation and is unconformably overlain by composed of boundstone containing spar-filled voids and possibly the Cane Hill Member of the Hale Formation (Morrowan). In stromatactis (Heckel 1972). Mound development is very localized in Washington County the unit crops out along a narrow northeast- the area and generally is confined to the top one-half or one-third of southwest-trending belt from Fayetteville to Evansville, Arkansas the formation. The mound core grades laterally into nodular (Fig. 1). Seventeen stratigraphic sections of Pitkin Limestone were limestone-shale beds, orintobioclastic and oolitic beds. measured throughout the study area to delineate lithofacies within the formation and to determine their regional geometry. Lithofacies determinations involved both field observations and thin-section DEPOSITIONALHISTORY examination. Five major facies were delineated withinthe formation: (1) oolith facies, (2) bioclast facies, (3) nodular limestone-shale Chesterian seas apparently encroached across northwest Arkansas facies, (4) mudstone facies, and (5)lime mud mound facies (Fig. 2). from the southeast (Fig. 3). A decrease in Fayetteville terrigenous sedimentation was succeeded by Pitkin carbonate deposition. Examination of the Fayetteville-Pitkin contact indicates that LITHOFACIES sedimentation was continuous across the boundary. Oolith shoals withassociated spillover lobes migrated across the platform as Pitkin Oolitic units are usually thick to massive-bedded packstone and sedimentation commenced. Coalescing oolitic and skeletal blanket grainstone ranging in thickness from fourinches to 11 feet. The beds sand bodies were deposited adjacent to the shoals in this initial commonly show a blocky weathering pattern. These oolitic strata are phase of Pitkin sedimentaton. Subsidence of the platform and present without restriction in the measured sections, but are most possibly eustatic oscillations of sea levelprovided the mechanism for common in the lower one-third of the formation and in the inter- abrupt facies changes. Throughout most of the deposition of Pitkin mound areas. Oolith sand initiallyaccumulated in shoal areas across Limestone, sedimentation probably equaled subsidence. Differential the platform and eventually coalesced to produce blanket sand subsidence as a result of compaction of Fayetteville Shale formed bodies. These beds grade intoboth bioclastic and mudstone units. deeper or more protected areas across the platform in which a Bioclastic units for the most part are thick to massive-bedded, middle phase ofPitkin deposition occurred. These areas allowed the although thinner beds also are present. They are composed of deposition of mudstone and nodular limestone-shale. Itwas in these wackestone, packstone, and grainstone. The beds commonly have areas that mound growth commenced. Fistuliporid and fenestrate horizontal partings whichimpart a gnarled appearance to the units; bryozoans as well as Girvanella established themselves on the small they closely resemble the lumpy, gnarled mound units. Most of the mud mounds. Crinozoans probably attached to the flanks of the constituents of these beds are crinozoans, bryozoans, and mounds, thus holding the mud together. Early inorganic cementation brachiopods. Crinozoans appear to be the most dominant particle. also may have caused diagenetic lithification of the mound (Heckel Near the lime mud mounds bryozoans are dominant. Bioclastic beds 1974). Mound growth flourished during the final phase of Pitkin grade laterallyinto allof the other lithofacies. sedimentation inwhich more turbulent conditions prevailed. Mound Nodular limestone-shale units range from six inches to eight feet in growth increased because of better circulation patterns and thickness. They range from lenticular beds to limestone nodules moderate current and wave activity which stimulated faunal growth embedded in a shale matrix. The beds are composed of oncolith- and carbonate mud production, possibly bygreen algae. The mounds intraclast mudstone and bioclastic wackestone and mudstone. The were self-perpetuating features because the communities of orga- shale ranges from abundant partings to thin beds or stringers. This nisms trapped their own skeletal debris and the mud that they pro- facies may be the flank facies of the lime mud mounds and may form duced. Bioclastic and oolitic sands also formed inbroad intermound a transition zone between the mounds and the intermound areas. areas and eventually coalesced to form extensive blanket sand bodies Some of the material actually may have been derived from the lime during the final phase of Pitkin sedimentation. At the end of Pitkin mud mounds as a result of wave erosion. These strata grade laterally deposition, regression eventually was initiated, causing subaerial into mounds and also intobioclastic intermound strata. exposure and erosion ofPitkinand possibly post-Pitkin strata.

88 Arkansas Academy of Science Proceedings, Vol. XXX,1976 Published by Arkansas Academy of Science, 1976 88 Journal of the Arkansas Academy of Science, Vol. 30 [1976], Art. 35

Robert E. Tehan

LITERATURECITED ACKNOWLEDGEMENTS

HECKEL, P.H. 1972a. Possible inorganic origin for stromatactis in Appreciation is extended to the American Association of calcilutite mounds in the 'Fully Limestone, Devonian of New Petroleum Geologists for monies provided for this study York.Jour. Sed. Petrology, 42:7-18. from the Hugh D.Miser Memorial Fund. Appreciation also is extended to D. Zachry, W. Manger, K.C. Jackson, and HECKEL, P.H. 1972b. Recognition of ancient shallow marine P.H. Heckel who helped criticize and provide ideas for this environments. Pages 226-286 inRecognition of ancient sedimen- study by lengthy discussions and visits to Pitkin Limestone tary environments. SEPM Spec. Pub. 16. outcrops. HECKEL, P.H. 1974. Carbonate buildups in the geologic record: a review. Pages 90-154 inReefs in time and space. SEPM Spec. Pub. 18. TEHAN, R.E. 1976. Sedimentary petrology of the Pitkin Limestone (Chesterian): Washington and Crawford Counties, Arkansas. Unpublished M.S. thesis, University of Arkansas, 160 pp.

Figure 3. Isopachous of the Pitkin Formation (from Tehan Figure study map 1.Location of area innorthwest Arkansas. 1976).

Figure 2. Lithofaeies cross-section of the Pitkin Formation (from Tehan 1976). Vertical scale: 1inch =50 feet. Academy Proceedings, XXX, Arkansas of Science Vol. 1976 89 http://scholarworks.uark.edu/jaas/vol30/iss1/35 89